Navien E351 Fault Code: Causes, Fixes & Repair Costs
What does the Navien E351 fault code mean?
The E351 code appears on Navien combi boilers when the central heating circuit cannot reach or hold its target pressure within an acceptable timeframe. Specifically, the boiler monitors how long the built-in auto feeder valve stays open while attempting to top up the system. If that valve remains open for more than five minutes without the heating circuit reaching its required pressure — typically around 12–15 PSI (roughly 0.8–1.0 bar) when cold — the boiler shuts down and displays E351. This is distinct from the domestic hot water side, which runs at mains pressure; E351 concerns only the sealed heating circuit. The fault is the boiler's way of saying: 'I've been trying to refill the system for too long and something is stopping me from succeeding.'
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty or failing expansion vessel Common
Navien's own technical data suggests the expansion vessel is responsible for the majority of E351 faults. When the vessel's internal membrane deteriorates or the pre-charge pressure drops, the heating circuit experiences erratic pressure swings. The boiler interprets this as low pressure and keeps the auto feeder valve open trying to compensate, eventually timing out and throwing E351.
- Water leak within the heating system Common
Any slow or hidden leak — whether inside the boiler casing, at radiator valves, pipe joints, or underfloor pipework — will cause a gradual pressure drop. The auto feeder opens to compensate but cannot keep pace with the loss, triggering the fault. The heat exchanger is a specific area Navien engineers are directed to inspect for internal weeping.
- Stuck or faulty auto feeder valve Sometimes
The auto feeder valve itself may be stuck open, unable to close fully, or failing to open when required. If it is stuck open, water may be continuously draining into the system or overflowing; if stuck closed, the system cannot refill at all. Either condition leads to E351 without necessarily meaning there is a leak elsewhere.
- Closed or isolated cold water supply valve Sometimes
The auto feeder draws from the cold mains supply. If the isolating valve on the cold water feed has been partially or fully closed — perhaps after recent plumbing work — the feeder cannot draw water in to top up the circuit, causing the fill attempt to time out.
- Air trapped in the heating circuit Sometimes
Significant air pockets in the pipework or radiators can prevent the pressure sensor from reading accurately and may cause the system to behave as though it is under-pressurised. Unbled radiators, particularly on upper floors, are a common source of trapped air following a system drain-down or after radiators have been removed.
- Faulty pressure sensor or wiring fault Sometimes
If the pressure sensor is sending an incorrect low-pressure signal to the PCB, the boiler will believe the system needs topping up even when the actual pressure is fine. A damaged sensor, corroded connector, or loose harness can all produce a false E351. This is confirmed by checking whether the pressure gauge on the boiler front panel matches what the display reports.
- PCB failure Rare
In cases where all other components have been checked and found to be in order, the PCB itself may be misinterpreting sensor data or incorrectly controlling the auto feeder relay. PCB faults are relatively rare and are generally a diagnosis of exclusion after all other causes have been ruled out.
How to fix it
- Check the boiler's pressure gauge and compare it with the display reading DIY safe
Look at the physical pressure gauge on the front of the boiler. A cold heating system should normally sit between 1.0 and 1.5 bar (roughly 15–22 PSI). If the gauge shows normal pressure but the boiler is still displaying E351, this may indicate a faulty pressure sensor rather than a genuine pressure loss — note this down to tell your engineer. If the gauge shows low pressure (below 1.0 bar), move to the next step.
- Top up the system pressure using the filling loop if pressure is low DIY safe
Locate the filling loop — usually a braided flexible hose or a built-in lever valve beneath the boiler. Slowly open the valve and allow pressure to rise to around 1.2–1.5 bar, then close the valve completely. Do not overfill; stop before 2.0 bar. If you cannot find the filling loop or are unsure how the system works, do not attempt this and contact an engineer instead.
- Reset the boiler (no more than 2–3 times) DIY safe
Once pressure is at the correct level, press and hold the reset button as described in your Navien user manual. If the boiler fires up and runs normally, monitor the pressure over the next 24–48 hours. If pressure drops again within a short period, this strongly suggests a leak or a failed expansion vessel and you should not continue resetting — call an engineer.
- Inspect visible pipework, radiators, and the boiler casing for signs of water leaks DIY safe
Walk around your home and look for damp patches near radiators, beneath the boiler, along visible pipe runs, and around radiator valves. Check for water stains or corrosion on pipework. If you spot any active dripping or persistent moisture, do not attempt to fix it yourself — note the location and report it to your engineer.
- Bleed radiators, starting from the highest floor and working downwards DIY safe
If you suspect trapped air (for example, radiators feel cold at the top but warm at the bottom, or the system was recently drained), bleeding may help. Use a radiator bleed key to open the bleed valve on each radiator until water dribbles out steadily with no hissing air. Remember to check and re-top the system pressure afterwards, as bleeding releases a small amount of water.
- Check that the cold water supply isolating valve is fully open DIY safe
Locate the isolating valve on the cold water feed pipe to the boiler — this is typically found on or near the boiler itself and will have a flat-head screw slot or a lever handle. Ensure it is fully open (lever in line with the pipe, or slot in line with the pipe for a screw type). If you are unsure which valve this is, do not operate any valves and let your engineer identify it.
- Call a Gas Safe registered engineer to diagnose and repair the underlying fault Gas Safe engineer
If the fault persists after the homeowner checks above, or if pressure keeps dropping, a Gas Safe engineer is required. They will check the expansion vessel pre-charge pressure and replace the vessel if the membrane has failed, inspect internal boiler components including the heat exchanger for leaks, test and replace the auto feeder valve if stuck or malfunctioning, verify the pressure sensor and wiring harness, and carry out a full system pressure test to locate any hidden leaks. Do not repeatedly reset the boiler or ignore this fault — low pressure in a sealed system can affect boiler safety and efficiency.
Parts you may need
- Expansion vessel (replacement) · from £60
- Auto feeder valve (fill valve) · from £45
- Heating circuit pressure sensor · from £25
- Pressure sensor wiring harness · from £15
- Radiator bleed key (homeowner tool) · from £3
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£400, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the Navien E351 and topping up the pressure myself?
You can reset the boiler and top up the pressure once or twice as an initial check, but you should not make a habit of it. If the pressure keeps dropping and E351 keeps returning, there is an underlying problem — most likely a leak or a failed expansion vessel — that will not resolve itself. Repeatedly resetting while ignoring the root cause risks further damage and in the case of a leak, water damage to your property. Treat recurring E351 as a sign to book an engineer promptly.
How much does it cost to fix Navien E351 in the UK?
For the most common causes, most homeowners pay between £150 and £400 all in. An expansion vessel replacement typically costs £200–£350 including labour, while a faulty auto feeder valve or pressure sensor usually comes in at £150–£280. If the fault turns out to be a heat exchanger leak or a PCB failure, costs can rise to £400–£600 or more — at that level it is worth comparing the repair cost against a new boiler, particularly if your Navien is over ten years old.
Why does the Navien E351 fault relate to the expansion vessel so often?
Navien's own technical data indicates that expansion vessel failure accounts for a large proportion of E351 faults. The expansion vessel contains a pressurised air chamber separated from the heating water by a rubber membrane. Over time the membrane degrades or the pre-charge air pressure drops, meaning the vessel can no longer absorb the normal pressure changes as the system heats and cools. This causes the system pressure to swing unpredictably, prompting the auto feeder to activate repeatedly and eventually time out. Expansion vessels have a typical lifespan of 8–12 years and are one of the more commonly replaced components on maturing boiler systems.
Is E351 dangerous — should I stop using my boiler immediately?
E351 is a lockout code, meaning the boiler has already shut itself down as a precaution, so the immediate safety risk is low. However, you should not simply force it back into operation and ignore the fault. If the cause is a water leak inside the boiler, continued operation could worsen the leak or damage electrical components. If you can see any active dripping from the boiler or pipes, turn the boiler off at the mains and call an engineer without delay. If there is no visible leak and the boiler runs normally after a single reset and pressure top-up, it is reasonable to use it while awaiting an engineer appointment — but monitor the pressure daily.